What is the name of the thread compound that is usually applied with a brush?

The correct answer is pipe dope, which is a type of thread compound commonly used in plumbing and instrument fitting applications. Pipe dope is designed to seal pipe threads, providing a watertight and airtight seal when two threaded fittings are connected. This compound is typically applied with a brush or directly from a tube, allowing for an even and controlled application on the threads.

Pipe dope also serves the function of lubricating the threads to make assembly easier, which is particularly important in preventing damage to the threads during installation. Its adhesive properties help ensure that the seal remains intact even under various temperature and pressure conditions.

In contrast, plumbing putty is primarily used for sealing joints in sinks and bath fittings rather than threaded pipe connections. PTFE tape, often recognized as Teflon tape, is another thread sealing option, but it is usually wrapped around the threads rather than applied with a brush. Pipe chalk, on the other hand, is used for marking but does not serve to seal or lubricate threaded connections. This context helps illustrate why pipe dope is the preferred choice for sealing threaded joints in various piping systems.